Using Scoped Slots in Vue.js to Abstract Functionality

Let’s start with a short introduction to Vue.js slots concept. Slots are useful when you want to inject content in a specific place of a component. Those specific places that you can define are called slots.

For example, you want to create a wrapper component that is styled in a specific way but you want to be able to pass any content to be rendered inside that wrapper (it might be a string, a computed value, or even another component).

There are three types of slots:

default / unnamed slots: used when you have a single slot in a component. We create them by adding <slot> in the template where we want to be able to inject our content. This <slot> tag will be replaced with any content passed to the component’s template.

named slots: used when you have multiple slots in a component and we want to inject different content in different places (slots). We create those by adding <slot> with a name attribute (e.g. <slot name="header"></slot>). Then when we render our component, we provide a slot content for each named slot by adding a slot attribute with the slot name.

By doing that, the <slot> tags in the component will be replaced by content passed to the component.

scoped slot: used when you want a template inside a slot to access data from the child component that renders the slot content. This is particularly useful when you need freedom in creating custom templates that use the child component’s data properties.

Real-World Example: Creating a Google Map Loader component

Imagine a component that configures and prepares an external API to be used in another component, but is not tightly coupled with any specific template. Such a component could then be reused in multiple places rendering different templates but using the same base object with specific API.

So far, so good. With all that done, we could continue adding the other objects to the map (Markers, Polylines, etc.) and use it as an ordinary map component.

But, we want to use our GoogleMapLoader component only as a loader that prepares the map — we don’t want to render anything on it.

To achieve that, we need to allow the parent component that will use our GoogleMapLoader to access this.google and this.map that are set inside the GoogleMapLoader component. That’s where scoped slots really shine. Scoped slots allow us to expose the properties set in a child component to the parent component. It may sound like an inception, but bear with me one more minute as we break that down further.

2. Create component that uses our initializer component

In the template, we render the GoogleMapLoader component and pass props that are required to initialize the map.

3. Expose google and map properties to the parent component by adding a scoped slot

Finally, we can add a scoped slot that will do the job and allow us to access the child component props in the parent component. We do that by adding the <slot> tag in the child component and passing the props that we want to expose (using v-bind directive or :propName shorthand). It does not differ from passing the props down to the child component, but doing it in the <slot> tag will reverse the direction of data flow.

Now, when we have the slot in the child component, we need to receive and consume the exposed props in the parent component.

4. Receive exposed props in the parent component using the slot-scope attribute

To receive the props in the parent component, we declare a template element and use the slot-scope attribute. This attribute has access to the object carrying all the props exposed from the child component. We can grab the whole object or we can de-structure that object and only what we need.

Even though the google and map props do not exist in the TravelMap scope, the component has access to them and we can use them in the template.

Yeah, OK, but why would I do things like that? What is the use of all that?

Glad you asked! Scoped slots allow us to pass a template to the slot instead of a rendered element. It’s called a scoped slot because it will have access to certain child component data even though the template is rendered in the parent component scope. That gives us a freedom to fill the template with custom content from the parent component.

5. Create factory components for Markers and Polylines

Now, when we have our map ready, we will create two factory components that will be used to add elements to the TravelMap.

And we’re done!

With all those bits and pieces completed, we can now re-use the GoogleMapLoader component as a base for all our maps by passing different templates to each one of them. Imagine that you need to create another map with different Markers or just Markers without Polylines. By using a pattern of scoped slots, it becomes very easy since all we need to pass now is different content to the GoogleMapLoader component.

This pattern is not strictly connected to Google Maps; it can be used with any library to set the base component and expose the library’s API that might then be used in the component that summoned the base component.

It might be tempting to create a more complex or robust solution, but this gets us the abstraction we need and it becomes an independent piece of our codebase. If we get to that point, then it might be worth considering extraction to an add-on.